This invention relates to a radio architecture, and in particular to a CMOS architecture for a digital radio transceiver.
It is necessary, in the field of digital mobile telephony, to transmit and receive radio signals which carry digital signals. Moreover, it is preferable that the mobile transceiver should be as small and light as possible, with low power requirements. It is advantageous to realise the digital components of the transceiver, such as a digital signal processor and an A/D converter and a D/A converter, using CMOS manufacturing techniques. This means that it is also advantageous, from the manufacturing point of view, to realise the analog components of the transceiver, such as amplifiers, mixers, etc, using the same CMOS manufacturing techniques. An architecture of this type is disclosed in xe2x80x9cA Low-Power CMOS Chipset for Spread-Spectrum Communicationsxe2x80x9d, S. Sheng, et al, International Solid-State Circuits Conference, 1996.
However, CMOS transistors are normally designed to function as switches with low leakage currents. A consequence of this is that such transistors are less suitable for use in analog RF circuits. For example, they typically have low transconductances, especially at low bias voltages, resulting in low gain and high (phase) noise.
U.S. Pat. No. 5,407,849 discloses a method of manufacturing a CMOS circuit in which the threshold voltage of some of the transistors (FET""s) is reduced, for example to be close to zero volts.
Thus the prior art radio architectures involve compromising the performance of the device, if it is decided to use CMOS processes to realise the whole of the circuits. Meanwhile, U.S. Pat. No. 5,407,849 discloses reducing the threshold voltage of some of the FET""s in a CMOS circuit, but fails to disclose how this might have any application to radio architectures.
The invention involves using transistors with different threshold voltages in different parts of an integrated circuit for a digital radio.
Advantageously, the invention involves using transistors with high or normal threshold voltages in the circuits which handle the digital signals, and transistors with reduced threshold voltages in the circuits which process the analog signals.
In addition, the invention may also involve using some transistors with high or normal threshold voltages and some transistors with reduced threshold voltages in the front-end circuits of a radio transceiver. Such a transceiver may be a CMOS arrangement, or may use only NMOS or PMOS devices.